A spark plug is assembled to, for example, a combustion apparatus such as an internal combustion engine (an engine). In this case, the spark plug is used to ignite an air-fuel mixture in a combustion chamber. Generally, the spark plug includes an insulator having an axial hole, a center electrode, a tubular metal shell main body, and a ground electrode. The center electrode is inserted into the tip end side of the axial hole of the insulator. The metal shell main body is provided on the outer periphery of the insulator. The ground electrode is sealed to the tip end portion of the metal shell. A gap between the ground electrode and the center electrode is a spark discharge gap.
The metal shell includes a flange seat portion and a thread portion for mounting. The seat portion projects radially outward, of the metal shell. The thread portion is disposed on the tip end side with respect to the seat portion. The thread portion is used to install the metal shell on the combustion apparatus. A thread root portion is disposed between the seat portion and the thread portion. A solid annular gasket may be disposed at the thread root portion (for example, see JP-A-2008-135370). When the spark plug is mounted in the combustion apparatus, the gasket ensures air tightness between the spark plug (the metal shell) and the combustion apparatus.
The spark plug, for example, may be mounted in the combustion apparatus in such a way that the ground electrode is present between the fuel injection device and the spark discharge gap. In this case, the injected fuel contacts the back surface of the ground electrode. There is a consequent concern that supply of the air-fuel mixture to the spark discharge gap will be hindered by the ground electrode, adversely affecting the ignitability. Therefore, a method like the following is considered. Namely, a relative formation position of a thread ridge of the thread portion relative to a portion where the ground electrode is secured, which is at the tip end portion of the metal shell, is a position corresponding to, for example, where the tapping of an internal thread formed in a mounting hole of the combustion apparatus starts. Accordingly, when the spark plug is mounted in the combustion apparatus, the ground electrode is arranged in a predetermined position.